Stacked LCD unit

ABSTRACT

A LCD unit includes a plurality of LC panels stacked one on another. If a picture to be displayed on the LCD unit is a still picture, one of the LC panels consecutively scans plurality of rows of pixels in a direction opposite to the scanning direction of the rows of pixels in the rest of LC panels. If the picture is a moving picture, all the LC panels consecutively scans in the same direction.

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2007-193453 filed on Jul. 25, 2007, thedisclosure of which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

The present invention relates to a stacked liquid crystal display (LCD)unit and, more particularly, to a direct-view LCD unit including aplurality of LC panels stacked one on another. The present inventionalso relates to an electronic display system including such adirect-view LCD unit.

BACKGROUND ART

LCD devices have the advantages of achieving a lower power dissipationand a higher resolution, and thus are used in a variety of applicationsfrom a small-size portable telephone to a large-size television (TV)monitor. However, the contrast ratio of a LCD device alone is as low as1000:1 at most, which is far lower than the contrast ratio of the otherdisplay devices such as a CRT, a plasma display device which is alsoused as a TV monitor, ad a discharge-type display device referred to asfiled-emission display (FED) device or surface-emission display (SED)device which has a contrast ratio of around tens of thousands. Thus, ithas been pointed out that the LCD device has the problem of poorimage-expression capability upon display of an image having a darkpicturesque portion such as an image included in a movie.

For solving the above problem, a technique has been developed whereinthe light intensity of a backlight source of the LCD device iscontrolled depending on the image to be displayed, without improving thecontrast ratio of the LCD device itself, to thereby improve the contrastratio of the LCD device as a whole. However a cold cathode tube isgenerally employed in a surface-emitting backlight source, the coldcathode tube having a narrow dynamic range of the luminance. Thus, evenif the light intensity of the backlight source is controlled dependingon the image to be displayed, the improvement of the contrast ratioremains on the order of 2000:1 to 3000:1 at most. In addition, the coldcathode tube, which has an elongate shape, may suffer from a poorlycontrolled property of the luminance (brightness) in a partial screenarea if there are a bright area and a dark area at the same time on thescreen. This degrades the improvement of the contrast ratio obtained bythe luminance control of the backlight source. In short, the LCD devicesuffers from reduction in the effective contrast ratio upon display of alower-luminance area with a superior reproducibility on the screen,which also includes therein a higher-luminance area.

For avoiding the above problem, it is desired to markedly raise thecontrast ratio of the LCD device. However, the contrast ratio of the LCDdevice alone is around 1000:1 at most, as described before PatentPublications 1 and 2 describe a technique for improving the contrastratio of the LCD device as a whole without significantly improving thecontrast ratio of the LCD device itself. In this technique, a pluralityof LC panels are stacked one on another to form a stacked LCD unit,thereby reducing the black luminance and thus improving the contrastratio of the LCD unit as a whole. The term “black luminance” as usedherein means a luminance on a screen supplied with a gray-scale levelsignal of a lowest luminance, i.e., black signal.

FIG. 6 shows a typical LCD unit including two LC panels stacked one onanother. The LCD unit includes a first polarizing film 901, a first LCpanel 941, a second polarizing film 902, a second LC panel 942, and athird polarizing film 903, which are consecutively arranged from anoptical incidence side of the LCD unit. The first LC panel 941 includesa TN-mode liquid crystal (LC) layer 931, and a pair of transparentsubstrates 911 and 912 including transparent electrodes 921 and 922 inthe vicinity of the LC layer 931. The second LC panel 942 includes aTN-mode LC layer 932, and a pair of transparent substrates 913 and 914including transparent electrodes 923 and 924 in the vicinity of the LClayer 932. The transparent electrodes 921 and 923 of the LC panels 941and 942 are pixel electrodes to which a drive signal is supplied from adrive circuit 951, whereas the transparent electrodes 922 and 924 of theLC panels 941 and 942 are common electrodes.

If the contrast ratio of the LC panel 941, 942 alone is measured using alaser beam, the contrast ratio will be around 10 to 15. On the otherhand, if the overall contrast ratio of the LCD unit including the LCpanels 941, 942 is measured, the overall contrast ratio improves up toaround 100:1. If the contrast ratio of a LCD unit including three LCpanels is measured, the contrast ratio will improve up to around 1000:1,whereby a superior contrast ratio far exceeding the limit of thecontrast ratio of the LC panel alone will be achieved. The PatentPublications as described above are as follows:

Patent Publication 1—JP-1989-10223A

Patent Publication 2—JP-UM-1984-189625A

In the LCD unit described in Patent Publication 1, two LC panels 941,942 stacked one on another are used to achieve a higher contrast ratio,wherein the two LC panels are driven by a common signal supplied from asingle signal source. In this configuration, corresponding positions(pixels) of the two LC panels overlapping each other are driven by thesame signal at the sane timing irrespective of whether the picture to bedisplayed therein is a still picture or a moving picture. This may causeoverlapping of the transmittance change of both the LC panels in a frameperiod from the start of writing the image data to the end of holdingthe image data, thereby amplifying the amount of transmittance change orflicker in the frame period. The amplified flicker incurs the problem ofsense of discomfort in an observer of the LCD unit.

SUMMARY OF THE INVENTION

In view of the above problem in the stacked LCD unit as described above,it is an object of the present invention to provide a stacked LCD unitincluding a plurality of LC panels stacked one on another, which iscapable of suppressing the flicker occurring therein.

The present invention provides a liquid crystal display (LCD) unitincluding; a plurality of liquid crystal (LC) panels each including anarray of pixels, the LC panels being stacked one on another so thatcorresponding pixels of the LC panels are aligned with one another fordisplay of a picture; and a backlight source emitting light onto a rearside of the stacked LC panels, wherein; a plurality of rows of thepixels of at least one of the LC panels are scanned consecutively in afirst scanning direction from a first row of the pixels, and a pluralityof the pixels of rest of the LC panels are consecutively scanned in asecond scanning direction from a second row of the pixels; and the firstscanning direction is opposite to the second scanning direction, and/orthe first row is different from the second row.

The present invention also provides a liquid crystal display (LCD) unitincluding: a plurality of liquid crystal (LC) panels each including anarray of pixels, the LC panels being stacked one on another so thatcorresponding pixels of the LC panels are aligned with one another; abacklight source emitting light onto a rear side of the stacked LCpanels; and a mode switching section for selecting one of a first modewherein a plurality of rows of the pixels of the LC panels are scannedconsecutively in the same direction from the same row, and a second modewherein a plurality of rows of the pixels of at least one of the LCpanels are scanned consecutively in a first scanning direction from afirst row of the pixels, a plurality of the pixels of rest of the LCpanels are consecutively scanned in a second scanning direction from asecond row of the pixels, and the first scanning direction is oppositeto the second scanning direction, and/or the first row is different fromthe second row.

The above and other objects, features and advantages of the presentinvention will be more apparent from the following description,referring to the accompanying drawings.

FIG. 1 is a block diagram of an electronic display system according to afirst embodiment of the present invention.

FIG. 2 is a block diagram of an electronic display system according to asecond embodiment of the present invention.

FIG. 3 is a sectional view of the LCD unit used in the electronicdisplay system of FIGS. 1 and 2.

FIG. 4 is a schematic sectional view of the LCD unit of FIG. 3, showingthe optical path from the LCD unit to an observer.

FIG. 5 is a functional block diagram of the IC driver in the electronicdisplay system of FIG. 1.

FIG. 6 is a typical LCD unit including a plurality of LC panels stackedone on another.

EXEMPLARY EMBODIMENTS

Now, exemplary embodiments of the present invention will be describedwith reference to accompanying drawings, wherein similar constituentelements may be designated by similar reference numerals.

FIG. 1 shows a electronic display system (LCD system) according to afirst embodiment of the present invention. The LCD system includes animage source unit 100, a signal processing unit 110 and a stacked LCDunit 130 which are interconnected via cables 103, 116, 117. The stackedLCD unit 130 includes a par of LC panels 133, 136 stacked one onanother.

The image source unit 100 includes an image source 101 and a transmitter102. The image source 101 outputs a picture (image data) to be displayedon the LCD unit 130. The transmitter 102 converts the image data outputfrom the image source 101 into an image signal suitable fortransmission, and delivers the image signal to the signal processingunit 110 via cable 103. The image signal delivered to the signalprocessing unit 110 from the image source unit 100 includes a series ofsignals (serial signal), which is delivered from the signal processingunit 110 to the pair of LC panels 133, 136. However, the serial signaldelivered to the first LC panel 133 is different from the serial signaldelivered to the second LC panel 136.

THC63DV164 chip supplied from Xilinx Electronics Corp., for example, maybe used as the transmitter 102. In this case, the transmitter 102converts the parallel data output from the image source 101 into aserial signal, and delivers the converted serial signal to the signalprocessing unit 110 via a cable, e.g., telecommunication cable 103.However, since a typical DVI output suitable to a digital interface of apersonal computer may be used, the transmitter 102 is not restricted tothe above chip, and may be one which can output an equivalent signal.The image source unit 100 may be a personal computer having a typicalDVI output. The signal transmission may be performed between anytransmitter and a corresponding receiver, wherein the signal fortransmission may be other than the DVI output, and may be a digitalsignal transmitted via a plurality of digital signal cables having asuitable transmission rate or may be an analog signal.

The signal processing unit 110 includes a receiver 111 and a signalcontrol chip 112. The receiver 111 receives the image signal transmittedfrom the image source unit 100. The signal control chip 112 includes asignal generating section 113, a moving-picture judgment section 114,and a mode switching section 115. The signal generating section 113performs image processing onto the signal received by the receiver 111,to generate a pair of pictures to be displayed on the two LC panels 133and 136 of the LCD unit 130. The signal generating section 113 deliversthe signals generated by the image processing to the LC panels 133 and136 via signal cables 116 and 117. The signal delivered via the signalcables 116 and 117 are LVDS (low-voltage differential-signaling) signal,for example.

The signal control chip 112 mounted on the signal processing unit 110may be of a Stratix series, or FPGA (field programmable gate array),supplied from Altera Corp. Since this chip also has a LVDS transmitterfunction, use of this chip provides a simple circuit configuration. Itis to be noted however that the built-in memory of this chip has asmaller capacity, and it is preferable that an external buffer memory beprovided for this chip depending on the situation. Use of the FPGA meansthat the logic circuit is configured by a static RAM. Therefore, theconfiguration of the logic circuit should be provided from a ROM deviceprovided outside the chip upon turn ON of the chip. The reason for usingthe FPGA is that the FPGA is especially suitable for use in a testpurpose or a small-lot production. Types of the FPGA include an EEPROMcapable of being rewritten electrically, a PROM and a onetime PROM, allof which can be basically used for the present invention. If a large-lotproduction is intended, the logic circuit of the FPGA may be replaced bya gate array. On the other hand, if a large scale is intended, the FPGAmay be replaced by a full-custom LSI to obtain the advantage of thepresent invention.

The LCD unit 130 includes a first LC panel 133, a second LC panel 136,ad a light source 137, which are consecutively arranged from the frontside of the LCD unit 130. In the LCD unit 130, the first LC panel 133and the second LC panel 136 are stacked one on another, with an array ofpixels 138 of the first LC panel 133 being aligned with an array ofpixels of the second LC panel 136. Although FIG. 1 shows a single pixel138 including three (RGB) sub-pixels, a plurality of pixels are arrangedin an array. A plurality of scanning lines (not shown) extend in a rowdirection to consecutively scan respective rows of the pixels 138, and aplurality of data lines (not shown) extend in a column direction toprovide image data to respective columns of the pixels 138.

The light source 137 is provided on the rear side of the LCD unit 130far from the observer to supply backlight to the LC panels 133, 136. Thefirst LC panel 133 includes therein color filters, and is used as acolor LC panel, and the second LC panel 136 is used as a monochrome LCpanel. A vertical driver 131 and a horizontal driver 132 are providedfor driving the first LC panel 133, and another vertical driver 134 andanother horizontal driver 135 are provided for driving the second LCpanel 136.

In the LCD unit 130, the scanning direction of the first LC panel 133 isopposite to the scanning direction of the second LC panel 136. Morespecifically, the first LC panel 133 is scanned upward from the bottomrow of the pixels on the screen toward the top row of the pixels asdenoted by an arrow 141, whereas the second LC panel 136 is scanneddownward from the top row of the pixels on the screen toward the bottomrow of the pixels as denoted by an arrow 140. The scanning directions ofthe LC panels 133 and 136 are determined by the vertical drivers 131 and134, respectively.

The opposite scanning directions used in the present embodiment achievereduction of the noticeable flicker, the reason of which will bedescribed hereinafter. In the case of a TFT-driven LC panel, each pixelis driven by a constant-amount electric charge injected at a writetiming of the each pixel. In the write timing, the writing voltage Vs isexpressed using the amount of charge Qs provided to the pixel electrodeand the pixel capacitance C1, by the following simple formula:Vs=QS/C1.

If the amount of charge Qs remains as it is in each pixel until the nextwrite timing, the write voltage Vs in the pixel will be maintained.However, the Qs decreases due to the leakage current of the TFT etc. byan amount Qr until the next write timing, to thereby reduce the initialpixel voltage Vs down to a final voltage Ve at the end of a frame. Thefinal voltage Ve is expressed by the following formula:Ve=(Qs−Qr)/C1In each pixel, the reduction of voltage by Vs−Ve changes thetransmittance of the LC layer to change the luminance or brightness ofeach pixel. The observer of the LCD unit notices the change of theluminance as a phenomenon of flicker.

In the structure of LCD unit wherein two LC panels are stacked forimproving the contrast ratio, the optical transmittance of the LCD unitis a product of the optical transmittance of the first LC panel by theoptical transmittance of the second LC panel. It is assumed here thatthe each LC panel has a transmittance change ΔL due to the flicker. TheΔL may be a positive or negative value depending on the drive scheme ofthe LC panel. If the transmittance of both the LC panels changes from Lto L-ΔL, then the overall transmittance change of the two LC panels isexpressed by the following formula:L ²−(L−ΔL)²=2L×ΔL−(ΔL)².The above formula means that the transmittance change of each LC panelcaused by a flicker, if any, is amplified to thereby degrade the imagequality of the LCD unit.

The present embodiment employs an opposite-scanning-direction modewherein the two LC panels 133, 136 in the LCD unit 130 are scanned inopposite directions. An example of the opposite-scanning-direction modeis such that a top pixel of the first LC panel 133 has a luminance of Ldue to immediately after the write operation, whereas a correspondingtop pixel of the second LC panel 136 has a luminance of L-^(Δ)L due toimmediately before the write operation. Therefore, the overalltransmittance change is expressed by the following formula:L2−L×(L− ^(Δ) L)=L ^(Δ) LComparing the overall transmittance change in the case of using the samescanning direction against the overall transmittance change in the caseof using the opposite scanning directions, the latter has a lower valueby a difference LΔL, assuming that (ΔL)² is negligible due to thesmaller order of value of ΔL.

The intensity of flicker was experimentally measured using a LCD unitmanufactured according to the above exemplary embodiment. Measurementwas first performed for the case of a comparative example using the samescanning direction, which revealed a flicker intensity above −30 dB.Measurement was then performed for the case of the embodiment using theopposite scanning directions, which revealed a flicker intensity lowerthan −30 dB. This experimental measurement showed the advantage of theopposite scanning directions over the same scanning direction.

In the case of using the opposite scanning directions for the two LCpanels, there is no defect in display of a still picture. On the otherhand, there is a problem to be solved in display of a moving picturewherein the image is changed between frames, as will be detailedhereinafter. The moving picture has a image changed between an n-thframe and an (n+1)th frame, and continues the change of image along theelapse of time. In the case of opposite scanning directions for the twostacked LC panels, after the data of n-th frame is written into thefirst LC panel 133 and second LC panel 136, the data of (n+1)th frame iswritten into both the LC panels. In this (n+1)th frame, the data of nthframe is replaced by the data of (n+1)th frame in the first LC panel 133consecutively from the bottom toward the top of the screen, whereas thedata of n-th frame is replaced by the data of (n+1)th frame in thesecond LC panel 136 consecutively from the top toward the bottom of thescreen.

At the initial stage of writing data during the (n+1)th frame, the dataof top row of the pixels in the first LC panel 133 is replaced by thedata of (n+1)th frame, whereas the data of top row of the pixels in thesecond LC panel 136 remains as the data of n-th frame. Similarly, thedata of bottom row of pixels in the second LC panel 136 is replaced bythe data of (n+1)th frame, whereas the data of bottom row of pixels inthe first LC panel 133 remains as the data of n-th frame. That is, inthe (n+1)th frame as well as the other frames, there is a discrepancybetween the image of the pixels in the first LC panel 133 and the imageof the corresponding pixels in the second LC panel 136, and thisdiscrepancy is removed in only a short time period that starts after allthe data of moving picture is replaced in a frame and ends before thereplacement of the data starts in the next frame, thereby degrading theimage of the moving picture.

As described above, the image quality of moving picture, whereindifferent image data are delivered for respective frames, is degraded bythe opposite scanning directions. This problem can be solved by usingthe same scanning direction upon display of a moving picture. Thus, theLCD unit of the present embodiment employs a first mode using the samescanning direction in display of the moving picture, and a second modeusing the opposite scanning directions in display of the still picture.The moving-picture judgment section 114 judges whether or not thedelivered image data is for a moving picture, and the mode switchingsection 115 changes the display mode between the first mode and thesecond mode based on the judgment by the moving-picture judgment section114. The mode switching section 115 selects the first mode, i.e., thesame scanning direction if the picture is a moving picture. FIG. 2 showsthe LCD unit of FIG. 1 operating in the same-scanning-direction mode fordisplay of a moving picture. In FIG. 2, both the first and second LCpanels 133 and 136 scan the pixels in the downward direction from thetop to the bottom of the screen as denoted by an arrow 140. Although thesame scanning direction provides an adverse influence on the flicker,the observer conceives a less amount of flicker upon observing themoving picture due to the change of the image between adjacent frames,compared to the case of observing a sill picture.

Back to FIG. 1, the mode switching section 115 delivers a signalindicating whether the vertical driver 131 of the first LC panel 133should scan the pixels in the downward direction or in the upwarddirection depending on the selected mode. The vertical driver 131 has aconfiguration wherein the scanning direction can be switched between theupward direction ad the downward direction, and determines the scanningdirection based on the signal delivered from the mode switching section115. The signal control chip 112 switches the order of the image datatransmitted from the signal generating section 113. The signal controlchip 112 transmits the image signal sequentially from the top pixelstoward the bottom pixels if the scanning direction is the downwarddirection, whereas transmits the image signal sequentially from thebottom pixels toward the top pixels if the scanning direction is theupward direction.

The moving picture judgment section 114 judges whether or not thepicture is a moving picture based on the degree of similarity ordifference of image data on the entire screen between the image of n-thframe and the image of (n+1)th frame received from the image source unit100, by comparing these images. More specifically, a frame memoryprovided in the signal processing unit 100 stores therein the image dataof a precedent frame, which is compared against the data of the presentframe, to judge the degree of difference in the entire frame data. Ifthe degree of difference is higher than or lower than a threshold, thesignal processing unit 110 judges that the received picture is a movingpicture or a still picture. The judgment may be performed by comparingtwo consecutive frame data, or may be performed by comparing three ormore than three consecutive frame data.

Judgment as to the moving picture/still picture may use a singlethreshold that divides the moving picture from the still picture;however, this may be difficult in the case of a still picture having avariation in the vicinity of the threshold. Thus, it is preferable touse two different values for the threshold, including a larger one(threshold A) for judgment of the moving picture and a smaller one(threshold B) for judgment of the still picture. In this case, thesignal control chip 112 judges that the picture is a moving picture ifthe difference is above or equal to the threshold A, whereas judges thatthe picture is a still picture if the difference is equal to or belowthe threshold B.

It is noted that since a judgment of difference between data of twoframes may be affected by noise, such as caused by movement of a mouse,cursor or window upon switching of the scanning direction, a frequent orperiodical switching of the scanning direction may occur. For avoidingsuch a frequent switching operation, it may be effective to provide ahysteresis in the time period before switching. More specifically, forexample, a switching from a still picture to a moving picture isperformed after the judgment thereof is continued for one second, orsixty frames, whereas a switching from a moving picture to a stillpicture is performed after the judgment thereof is continued for thirtyseconds, or 1800 frames.

The moving-picture judgment section 114 judges that the picture shiftedinto or remains in a moving picture, if the state wherein ¼ of thepixels, for example, among 640×480 pixels of a VGA-size screen ischanged continues for thirty seconds in succession. The moving-picturejudgment section 114 judges that the picture shifted into or remains ina still picture, if the state wherein the number of pixels changed isequal to or below 1/16 of the 640×480 pixels continues for one second insuccession. This judgment prevents the frequent switching between thestill picture and the moving picture from occurring, thereby allowingthe observer not to feel a sense of discomfort on the screen. It ispreferable that the switching itself be performed after a writeoperation for a frame during a vertical flyback period. The presentembodiment also employs this type of switching for the scanningdirection. If the switching is performed outside the vertical flybackperiod, however, there is no significant degradation in the imagequality because the image degradation occurs only in a single frameperiod and will hardly be conceived by the observer.

FIG. 3 shows a sectional view of the LCD unit 130 shown in FIG. 1. TheLCD unit 130 includes a polarizing film 201, a transparent substrate 211color filter layers 251 an orientation film 221, a LC layer 231, anorientation film 222, a transparent substrate 212, a polarizing film202, a polarizing film 203, a transparent substrate 213, an orientationfilm 223, a LC layer 232, an orientation film 224, a transparentsubstrate 214, a polarizing film 204, and a surface-emission lightsource 241, which are consecutively arranged from the light emittingside of the LCD unit. In the following description, a combination of thetransparent substrate 211, color filter layers 251, orientation film221, LC layer 231 orientation film 222, and transparent substrate 212 isreferred to as a first LC element 261, for the sake of convenience fordescription. A combination of the first LC element 261 and thepolarizing film 201 and polarizing film 202 sandwiching therebetween thefirst LC element 261 is referred to as the first LC panel 133. Similarlya combination of the transparent substrate 213, orientation film 223, LClayer 232, orientation film 224, and transparent substrate 214 isreferred to as a second LC element 262, and a combination of the secondLC element and the polarizing film 203 and polarizing film 204sandwiching therebetween the second LC element 262 is referred to as thesecond LC panel 136.

The surface-emission light source 241 in FIG. 3 corresponds to the lightsource 137 shown in FIG. 1. The surface-emission light source 241configures a planar light source emitting light from the planar surfacethereof, and emits light onto the first LC panel 133 and second LC panel136 through the rear side thereof. The light emitted from thesurface-emission light source 241 consecutively passes the second LCpanel 136 and first LC panel 133, to reach the observer. Control oftransmission of the light in the respective pixels of the first LC panel133 ad second LC panel 136 provides a desired image or picture to theobserver.

A fabrication process of the LCD unit will be described hereinafter.Fabrication of the first LC panel 133 will be first described. Thetransparent substrate 212 is provided with a matrix of electrode pairs,such as 923 shown in FIG. 6, on the surface of the transparent substrate212 near the LC layer 231. A three-terminal nonlinear device, such as aTFT, is provided for a pixel electrode of each of the electrode pairs,to associate each pixel with a corresponding TFT. Each LC panel operatesin an in-plain-switching (IPS) mode using a lateral electric field,generated by a pixel electrode and a corresponding common electrode eachconfiguring a comb-teeth electrode. The color filter layers 251 includea single stripe layer for each sub-pixel to represent one of threecolors, i.e., red(R), green(G) and blue(B) in the pixel. Threesub-pixels representing three different colors configure a single pixelin the first LC panel 133.

Orientation films 221, 222 are formed on the surface of the color filterlayers of the transparent substrate 211 and the surface of thetransparent substrate 212 on which the matrix of electrode pairs isformed, respectively, by coating and subsequent orientation processing,such as a rubbing treatment. The transparent substrate 211 andtransparent substrate 212 are then assembled with an intervention of aspecific gap so that the orientation film 221 and orientation film 222oppose each other with the orientation directions thereof being parallelwith or opposite to each other. The specific gap is then provided withLC such as ZLI4792 from Merck Co. by injection thereof, to configure thefirst LC element 261. The first LC element 261 is then sandwichedbetween the polarizing film 201 and the polarizing film 202 to configurethe first LC panel 133. The polarizing films may be SEG1224 from NittoDenko Corp. The polarizing films 201 and 202 are arranged so that thelight transmission axes or light absorption axes thereof areperpendicular to each other, and the light absorption axis of one of thepolarizing films 201 and 202 is parallel to the orientation direction ofthe LC layer 231.

The fabrication process for the second LC panel 136 will be thendescribed. This process is similar to the fabrication process for thefirst LC panel 133 except that the step of forming the color filterlayers is not needed for the second LC panel 136. More specifically, thetransparent substrate 214 is provided with a matrix of electrode pairs,such as 921 in FIG. 6, on the surface of the transparent substrate 214near the LC layer 232. A three-terminal nonlinear device, such as a TFT,is provided for a pixel electrode of each of the electrode pairs, toassociate each pixel with a corresponding TFT. Since the second LC panel136 does not include color filter layers, the size of each pixel of thesecond LC panel 136 may be designed to correspond to the size of thepixel of the first LC panel 133 including three sub-pixels. In analternative, the configuration of a single pixel of the second LC panel136 may correspond to the configuration of a sub-pixel of the first LCpanel 133, except for the absence of the color filter layers. The secondLC element 262 thus fabricated is sandwiched between the polarizing film203 and the polarizing film 204 to configure the second LC panel 136.The polarizing films 203 and 204 are arranged so that the lighttransmission axes or light absorption axes thereof are perpendicular toeach other, and the light absorption axis of one of the polarizing films203 and 204 is parallel to the orientation direction of the LC layer232.

The first LC panel 133 ad second LC panel 136 are then stacked one onother after alignment of corresponding pixels therebetween. Thesurface-emission light source 241 is then arranged on the rear side ofthe second LC panel 136 to configure the LCD unit 130. Upon stacking thefirst LC panel 133 onto the second LC panel 136, the orientationdirections thereof are arranged parallel or perpendicular to each otherso that the light passed by the polarizing film 202 may be passed by thepolarizing film 203 as much as possible. In an alternative, one of thepolarizing films 202 and 203 may be omitted, thereby allowing both theLC panels to share therebetween the other of the polarizing films 202and 203.

In the LCD unit 130 of the present embodiment, the color filter layers251 are provided only for the first LC panel 133. In this case, when thefield of view is physically moved parallel to the LC panels, thetransmission luminance is not substantially varied depending on theviewing angle, differently from a LCD unit including color filter layerson both the LC panels. However, it is to be noted that if both the LCpanels 133 and 136 are driven by the same signal supplied from a commonsignal source, the distance between the LC layer 231 and the LC layer232 causes a parallax which degrades the image quality of the LCD unitas will be detailed hereinafter.

FIG. 4 shows a simplified structure of the LC panels as described abovefor showing the problem of the parallax. In FIG. 4, only the transparentsubstrates and LC layers among the constituent elements in FIG. 3 aredepicted, wherein LC panels 301 and 302 correspond to the LC panels 133and 136, respectively, and LC layers 325 and 326 correspond to the LClayers 231 and 232, respectively.

Observation of the LC panels 301 and 302 by an observer 311 in the lineof sight denoted by numeral 331 that is perpendicular to the screen ofthe LCD unit allows a point a on the second LC panel 302 and acorresponding point β on the LC panel 301 to overlap each other. Thus,both the points α and β are observed as the same point in the line ofsight 331 by the observer 311. More specifically, observation in thenormal direction does not cause a parallax, thereby providingsubstantially no sense of discomfort to the observer 311. On the otherhand, observation of the LC panels 301 and 302 by an observer 312 in aviewing direction slanted from a perpendicular of the screen by an angleθ causes both the points α and β to be apart from each other due to thedistance “d” therebetween. The point α is observed in the sight of view332, whereas the point β is observed in the sight of view 333, whereby aparallax 334 is conceived by the observer 312. The parallax 334 causes asingle line or single point in the image data to be observed as doublelines or double points, to incur a sense of discomfort in the observer.

Light passed by the first LC panel 301 and second LC panel 302 isemitted through the transparent substrate 321 into the air, whereby thedirection of light is changed due to Snell's law depending on thedifference in the refractive index between the transparent substrate 321and the air. Assuming that θ, φ, ng and na are the emission angle oflight from the transparent substrate 321, incidence angle of light ontothe surface of the transparent substrate 321, refractive index of thetransparent substrate 321 and refractive index of air, respectively thefollowing formula:na×sin θ=ng×sin φholds based on the Snell's law. Transformation of the above formulaprovides the following relationship:φ=sin⁻¹((na/ng)×sin θ).From the relationship of alternate angle, the angle between the lightadvancing from the point β toward the surface of the transparentsubstrate 321 and a perpendicular of the transparent substrate 321 canalso be expressed similarly. The same applies to the point α. Theapparent, distance “r” between the point α and the point β on the LCpanel 301 as observed in a viewing direction of θ can be obtained asfollows:tan φ=r/dr=d×tan φ=d×tan(sin⁻¹((na/ng))×sin θ)  (1).

For removing the parallax in the slanted viewing direction, it iseffective to extend a point image, which is to be displayed originallyon the point β of the LC panel 301, to have a linear image having alength of r, as shown in FIG. 4. Thus, the present embodiment employs anaveraging processing wherein the information to be displayed on thepoint β is scattered onto the line segment between the point β and apoint γ which is apart from the point β by a distance “r”. The averagingprocessing reduces the sense of parallax to provide a comfortable imageon the LCD unit. The averaging processing may be performed to the imagedata of any of the LC panels 301 and 302. In the view point of reductionin the sense of parallax, an equivalent advantage can be obtained by theaveraging processing of image data for any of the LC panel 301 includingcolor filter layers ad the LC panel 302 including no color filterlayers. Similarly, an equivalent advantage can be obtained by theaveraging processing of the image data for any of the front-side LCpanel and the rear-side LC panel, the terms “front-side” and “rear-side”being used herein based on the standpoint of the observer.

If the averaging processing is performed onto the image data for therear-side LC panel, i.e., second LC panel 302, an optical component suchas a light diffusion film may be inserted between the first LC panel 301and the second LC panel 302. The optical component having a lightdiffusion function apparently enlarges the distance r′ which is obtainedby the averaging processing of the data for the LC panel 302 in theimage processing. The distance r′ may be expressed by the followingformula:r′=(d′×tan φ)+((d−d′)×(tan(φ+η)),where d′ and η are the distance between the optical component and the LClayer 326 and the diffusion angle (half-value-width diffusion angle),respectively. The image processing should preferably consider the effector this enlargement of the distance by the optical component.

The present inventors conducted investigation of the drive scheme forthe LCD unit including the two LC panels, and found a suitable drivescheme as detailed hereinafter. The suitable drive scheme is such thatthe averaging processing is performed onto the image data for a LC panel(such as 302) including no color filters, and this LC panel (302) isstacked onto another LC panel (such as 301) including color filterlayers. The reason for performing the averaging processing onto the datafor the LC panel including no color filters is that the averagingprocessing of the image data for the LC panel including color filtersmay mix the three colors to form a dull color tone and reduce thereproducible range of color.

FIG. 5 shows a functional block diagram of the signal control chip 112.The signal control chip 112 includes a monochrome-image generationsection 501, a first calculation section (averaging processing section)502, timing controllers 503, 505 and 506, a second calculation section504, a control-signal generation section 507, and transmitters 508 and509. The second calculation section 504 acts as the moving-picturejudgment section 114 shown in FIG. 1, which judges whether the deliveredpicture is a moving picture or a still picture. The control-signalgeneration section 507 acts as a mode switching section 115 whichperforms switching of the scanning direction for the first LC panel 133.The output signal of the second calculation section 504 is delivered tothe first LC panel 133 through transmitter 509 via timing controller505. The output signal from the first calculation section 502 isdelivered to the second LC panel 136 through transmitter 508 via timingcontroller 506.

Synchronization signals including V-Sync, H-Sync, Dot Clock for theimage data are input to the signal control chip 112 through the receiver111. The signal control chip 112 performs re-processing for the receivedsynchronization signals in the control-signal generating section 507 todeliver an internal synchronization signal for synchronization betweenthe same and each of the signal control chip 112 and LCD unit 130. Inthe present embodiment, since FPGA supplied from Altera Corp. is used asthe signal control chip 112, the storage capacity for temporarilystoring the image data is insufficient. Thus, an external storage unitconfigured by a SRAM is provided to the signal control chip 112.However, if the signal control chip 112 is configured by a full-customASIC or a DRAM-mounting ASIC, the image data can be stored in the ASICand the external storage unit may be omitted. Thus, the external storageunit is not depicted in FIG. 5. The timing controllers 503, 505 and 506and first and second calculation sections 502 and 504 each include astorage unit therein. The storage capacity needed for the storage unitof the timing controllers 503, 504 and 505 is relatively small, and thusa line buffer may be used for the storage unit of the timing controllers503, 504 and 505. The line buffer may be configured by a built-in bufferinstalled in FPGA.

The signal control chip 112 receives, for example, a 24-bit image datasignal including S-bit data for each prima color from the receiver 111.This image data signal is separated in the signal control chip 112 intoa color-image data signal displayed on the first LC panel 133 (FIG. 1)and a monochrome-image data signal displayed on the second LC panel 134.More specifically, the image data signal received by the signal controlchip 112 is delivered to the monochrome-image generation section 501which generates a monochrome gray-scale level signal, ad to the timingcontroller 503 which reads and outputs the signal in the order ofreceiving the image data signal based on the timing of the output side.

The monochrome-image generation section 501 generates an 8-bitmonochrome image signal based on the luminescence information of theinput 24-bit color image data signal. Generation of the monochrome imagedata is performed by judging the gray-scale level of each primary colordata (i.e., red, green and blue) for each pixel, and selecting a maximumgray-scale level among the gray-scale levels of the three primary colorsas the converted gray-scale level of the each pixel after conversion ofthe monochrome image data. In an alternative, a HSV conversion thatconverts the input image data to the information of brightness,chromaticness and chromaticity is performed, and only the information ofbrightness is extracted to configure the monochrome data. In a furtheralternative, one of the R, G and B color data may be selected for theconverted monochrome image data or two of the R, G and B color data maybe selected and converted into the monochrome data. It is to be notedthat the area or pixel having a higher gray-scale level (i.e.,transmittance) of the monochrome image data generally corresponds to anarea or pixel having a higher brightness or higher chromaticness.

The monochrome-image generation section 501, after converting the inputimage data into the monochrome image data, selects a specifictransmittance for a pixel having a transmittance equal to or above athreshold, and selects a transmittance corresponding to thetransmittance of the original color image data for a pixel having atransmittance below the threshold. In this processing, themonochrome-image generation section 501 compares the gray-scale level ofeach pixel of the converted monochrome image data against the thresholddetermined beforehand, to select the specific transmittance, preferablya full transmittance, if it is judged that the gray-scale level of thepixel is equal to or above the threshold. In the description to follow,the specific transmittance is assumed to be the full transmittance. Ifit is judged that the gray-scale level of the pixel is lower than thethreshold, the transmittance of the pixel is recalculated by using agray-scale level conversion to assume a corresponding value between thefull transmittance and a minimum transmittance.

The conversion processing wherein a gray-scale level above the thresholdis replaced by the full transmittance may be modified from the aboveprocessing. For example, a γ-curve conversion processing wherein theγ-value is round 4.0 may be used to allow pixels having a gray-scalelevel above a specific value to have a full transmittance. In analternative, a histogram conversion or adjustment may be used to allowpixels having a gray-scale level above a specific value to have a fulltransmittance. It is sufficient that the monochrome-image generationsection 501 obtain a monochrome image data wherein pixels having arelatively higher transmittance have a full transmittance, and thus thetechnique for generating the monochrome image data or the technique forconverting a relatively higher transmittance to the full transmittancemay be modified from the above example, or any other technique may beemployed instead.

The first calculation section 502 performs an averaging processing ontothe monochromatic image data generated by the monochrome-imagegeneration section 501. The averaging processing may use the techniquedescribed in Patent Publication JP-2007-286413A, the description ofwhich is incorporated herein in its entirety by reference. In thistechnique, the image data of pixels located within a specific distance,which corresponds to the distance “r” in FIG. 3, from a target pixel areaveraged. This averaging processing may employ a weighted averagingprocessing where the weighting factor depends on the distance from thecentral pixel. The weighting factor may be assumed to depend on theGaussian distribution. The averaging processing allows the monochromeimage data to have a dull edge in the picture. The monochrome image datasubjected to the averaging processing in the first calculation section502 is delivered to the second LC panel 136 via timing controller 506and transmitter 508.

The second calculation section 504 generates color image data for thefirst LC panel 133, based on the 24-bit color image data deliveredthrough timing controller 503 and monochrome image data subjected to theaveraging processing in the first calculation section 502. The secondcalculation section 504 performs calculation processing onto the 24-bitcolor image data based on the monochrome image data delivered from thefirst calculation section 502, to generate a color image signal. Morespecifically, the second calculation section 504 divides the input colorimage data by the brightness of the monochrome image data, to therebygenerate image data having a corrected brightness. However, if thebrightness of the monochrome image is at a zero level, the process ofdivision by zero is replaced by a exceptional procedure that isdetermined beforehand. In an alternative, the zero level of themonochrome image data is shifted toward a higher level by a specifiedlevel to avoid the division by zero. Upon generation of the color imagesignal in the second calculation section 504, a correction processingmay be additionally performed onto the original image signal. The colorimage data generated by the second calculation section 504 is deliveredto the first LC panel 133 through transmitter 509 and timing controller505.

The second calculation section 504 has a function of comparing the imagedata of the present frame against the image data of the previous frame,to judge whether or not there occurs a change of image data. If theimage of pixels in number corresponding to 5% of the total number ofpixels is changed, and this state continues for one second, the secondcalculation section 504 judges that the picture remains at or shifted tothe moving picture. If the number of pixels having a change in the imageis below 1% of the total number of pixels and this state continues for30 seconds, the second calculation section 504 judges that the pictureremains at or shifted to a still picture. The second calculation section504 notifies, upon detecting the shift to a moving picture or a shift toa still picture, the occurrence of the shift to the control-signalgeneration section 507. If a shift to the moving picture is notified,the control-signal generation section 507 instructs the vertical driver131 shown in FIG. 1 to switch the scanning direction to a downwardscanning direction. On the other hand, if a shift to the still pictureis notified, the control-signal generation section 507 instructs thevertical driver 131 to switch the scanning direction to an upwardscanning direction.

The second calculation section 504 instructs, upon detection of a shiftto the moving picture or still picture, timing controller 505 to switchthe order of transmission signal to an opposite order. Morespecifically, if the second calculation section 504 detects a shift tothe moving picture, timing controller 505 delivers the image data in anormal transmission order from the top row to the bottom row. On theother hand, if the second calculation section 504 detects a shift to thestill picture, timing controller 505 delivers the image data in areversed transmission order from the bottom row to the top row. Due tothis operation wherein the control-signal generating section 507 changesthe scanning direction of the vertical driver 131 and timing controller505 switches the transmission order of the image data, the first LCpanel 133 switches scanning direction between the upward scanningdirection and the downward scanning direction.

In the LCD unit 130 of the present embodiment, as described heretofore,the first LC panel 133 is driven by the color image data generated inthe second calculation section 504, whereas the second LC panel 136 isdriven by the monochrome image data generated by the monochrome-imagegeneration section 501 and subjected to the averaging processing in thefirst calculation section 502. If only the image of the second LC panel136 is observed, a portion of the screen receiving image data having arelatively higher brightness has a full transmittance and the rest ofthe screen has a dull image due to the averaging processing. On theother hand, if only the image of the first LC panel 133 is observed, aportion of the screen corresponding to the portion of the second LCpanel 136 having a dull image has an emphasized image that is emphasizedin the brightness and chromaticness thereof from the original colorimage data.

In the structure of FIG. 5, the control-signal generating section 507 isinstalled in the signal control chip 12 which also includes thereinsignal processing sections including the control-signal generatingsection 507 for generating a timing signal controlling the operation ofthe chip. It is to be noted however that the control-signal generatingsection 507 may be provided outside the signal control chip 112 toprovide an external control signal to the chip. In a furtheralternative, a plurality of signal control chips may be provided in theLCD system, wherein one of the signal control chips provides, togetherwith a data signal, a synchronizing signal generated therein to asucceeding one of the signal control chips.

In order to verify effects of the present embodiment, the LCD system ofFIG. 1 was operated to display a picture on the LCD unit 130 includingthe first LC panel 133 and second LC panel 136. The LCD unit 130provided superior brightness ad chromaticness comparable to thebrightness and chromaticness, respectively, of the first LCD panel 133used alone for display of an ordinary color image data. The LCD unit 130also provided a superior contrast ratio as high as 500,000:1, which thefirst LCD panel 133 alone could not provide. In a slanted viewingdirection, the LCD unit 130 provided a superior image qualitysubstantially without die influence by a parallax due to the averagingprocessing performed in the second LC panel 136. The first LC panel aswell as the second LC panel used alone had a contrast ratio of around700:1. If the LC panel alone has a contrast ratio of 1000:1, die LCDunit including two of this LC panel stacked one on another may provide ahigher contrast ratio as high as 1,000,000:1. Three or more of the LCpanel stacked one on another may provide a further higher level of thecontrast ratio, the magnitude of which an ordinary contrast-ratiomeasurement device cannot measure.

The example of LCD system shown in FIG. 1 included the image source unit100, signal processing unit 110, and LCD unit 130 which are providedseparately from one another; however, these units need not be providedas separate hardware and may be configured as a single member orincluded in a single housing. In an alternative, the LCD unit 130 may beseparated from the image source unit 100 and signal processing unit 110,which are integrated with each other or received in a single housing.The signal processing in the signal processing unit 110 may beperformed, instead of using hardware, by using software including aplurality of image processing programs running on a CPU and/or a signalprocessor.

In the above configuration, the first and second calculation section 502and 504 in the signal control chip 112 perform calculation processingfor the image data to thereby generate picture data. However, the signalprocessing need not be a calculation processing, and may use a lookuptable which defines the relationship determined beforehand bycalculation between the input image data and the output image data. Inthis case, for example, the monochrome-image generation section 501 mayinclude a selector for selecting a color sub-pixel that has a maximumbrightness among the three color sub-pixels, and output of the firstcalculation section 502 for performing the averaging processing of themonochrome imaged data is used for adjustment of brightness withreference to the lookup table. The lookup table may be asingle-dimensional table tabulating a set of input image signals and aset of output image signals. The advantage of using such a lookup tableis that the relationship between the input image data and the outputimage data may be arbitrarily defined therein to obtain a finebrightness adjustment. This suppresses occurring of a discontinuouspoint that incurs a problem in the calculation processing.

On the other hand, the second calculation section 504 uses the inputcolor data and a set of image data created in the first calculationsection 502. In this case, since at least one-dimensional image datagenerated in the first calculation section 502 is added to then-dimensional input image data, the second calculation section 504 usesa lookup table having (n+1)-dimensional image data that is one dimensionhigher than the n-dimensional image data. In the present embodiment, thesecond calculation section 504 uses a four-dimensional lookup table,which provides a gray-scale level of the color image data from thegray-scale level of the input RGB image data and gray-scale level of themonochrome image data generated by the first calculation section 502,thereby generating the color image data to be used in the first LC panel133.

In an alternative, the input RGB image data is subjected to a HSVconversion to generate separate data including hue, chromaticness andbrightness data. A set of the brightness data thus obtained and anotherset of monochrome image data created by the first calculation section502 are used as the base data to create a two-dimensional lookup table,whereby a new set of brightness data is generated. The new set ofbrightness data of the image signal is combined with the hue andchromaticness data separated before, to thereby generate a new RGBgray-scale level, which is input to the first LCD panel 133 via timingcontroller 505 and transmitter 509, to thereby display a color picture.In this case either, the lookup table is a two-dimensional lookup table,whereby the lookup table used in the second calculation section 504 isone dimension higher than the dimension of the lookup table used in thefirst calculation section 502.

In the above embodiment, it is unnecessary to install the lookup tablein both the first calculation section 502 and second calculation section504. In this case, the first calculation section 502 perform an ordinarylogic calculation processing (i.e., at zero dimension), and the secondcalculation section 504 uses a lookup table having two dimensions. Inthe present embodiment, the first LC panel 133 includes color filterlayers 251 (FIG. 3); however, the color filter layers are not anindispensable constituent element that solves the parallax problem inthe image data subjected to the averaging processing. Accordingly, thefirst LC panel 133 may be configured by a monochromatic LC panelsimilarly to the second LC panel 136, to provide a monochrome LCD unit.

In the above embodiment, the second LC panel 136 may have three pixelscorresponding to RGB color filter layers of the first LC panel 133.However, the color filter layers need not be RGB color filters, and maybe other primary color filters, or may be multiple-color filtersincluding RGBYMC, for example. In such a case, a single pixel may bedivided into a plurality of sub-pixels corresponding to a desired numberof multiple-color filters. A single pixel may be divided into foursub-pixels such as R, G, G and B, or into four sub-pixels correspondingto three color filters and including an additional colorless (W)sub-pixel.

Although an IPS-mode LCD unit is exemplified in the above embodiment,the drive mode of the LCD unit may be a vertical alignment (VA) mode,twisted nematic (TN) mode, optically-compensated-bent mode etc. In theexample of FIG. 3, a retardation compensating film is not providedtherein; however, a retardation compensating film may be providedbetween the first LC element 261 and each of the polarizing films 201and 202, and between the second LC element 262 and each of thepolarizing films 203 and 204, for improving the viewing anglecharacteristic. Upon insertion of the retardation compensating film, theoptical characteristic of the retardation compensating film may bedetermined depending on the mode of the LC layer to be employed, as willbe detailed below.

If a retardation compensating film is inserted within the first LC panel133 in case of the first LC panel 133 being driven in an IPS mode, it iseffective to insert the retardation compensating film between thepolarizing film 201 and the first LC element 261 and between thepolarizing film 202 and the First LC element 261. Assuming that theretardation compensating film has a maximum refractive index of nx, arefractive index of ny in the direction normal to the direction of nxwithin the substrate plane, and a refractive index of nz in thedirection normal to the direction of nx and ny, it is preferable thatthe retardation compensating film has a characteristic of. nx≧nz>ny andbe arranged so that the direction of nx is parallel to the opticalabsorbing axis or optical transmission axis of the polarizing films 201and 202. This arrangement improves the viewing angle characteristic ofthe first LC panel 133. The retardation compensating film may include aplurality of layers having an overall characteristic satisfying theabove relationship for the refractive indexes nx, ny and nz.

If the first LC panel 133 is driven in a VA mode, it is preferable thatthe retardation compensating film having a relationship of nx≧ny>nz isarranged so that the direction of nx is parallel to the opticalabsorption axis or optical transmission axis of the polarizing films 201and 202 to improve the viewing angle characteristic. If the LC panel isdriven in a TN mode or OCB mode, it is preferable that the retardationcompensating film be configured by a wide view (WV) film including adiscotic LC layer that has a negative retardation and an optical axishaving a direction varying with the thickness-wise position thereof.This retardation compensating film also improves the viewing anglecharacteristic.

The retardation compensating film may be inserted only at one side ofeach of the LC elements 261 and 262, or may be inserted on both sidesthereof. Although the above description is such that the retardationcompensating film may be inserted between each of the LC elements 261and 262 and the associated polarizing films 201-204, the retardationcompensating film may be inserted at any position between each of the LClayers 231 and 232 and the associated polarizing films 201-204 at anylocation. In additions the retardation compensating film may include aplurality of layers. Further, the above description premises that theoptical transmittance of the second LC panel 136 has a constant value solong as the pixel has a transmittance above the threshold; however, theoptical transmittance may have a range of variation so long as the rangeof variation is within several percents of the total transmittance.

In the present embodiment, the description exemplifies a verticalscanning direction; however, the scanning direction is not limited tothe vertical direction, and may be a horizontal direction or even aslanted direction depending on the circumstances. In the presentembodiment, the scanning direction is switched depending on the movingpicture or still picture. However, if the most of the pictures to bedisplayed is a moving picture as in the case of a TV set, the scanningdirection may be fixed to the same direction in the LC panelsconfiguring the LCD unit. If the most of the pictures is a still pictureas in the case of a LCD unit used for display in an art museum or for anX-ray image, the scanning direction may be fixed to opposite scanningdirections. For fixing the scanning direction, it is sufficient to fixthe result of judgment as to the still picture or moving picture to oneof the still picture and moving picture.

In the present embodiment, the first LC panel 133 and second LC panel136 have the same resolution; however, both the LC panels may have somedifference therebetween in the resolution. The first LC panel 133disposed at the front side may have a higher resolution than the secondLC panel 136 disposed at the rear side, and vice versa. If both the LCpanels have different resolutions, it is preferable that a signalconversion processing be performed before the signal processing by themonochrome-image generation section 501 in the signal control chip 112.If the monochrome image signal delivered from the first calculationsection 502 is used in the second calculation section 504, it ispreferable at the resolution be converted for adapting to the secondcalculation section 504.

In the above example, the signal control chip 112 was configured by aFPGA1 chip for performing the verification for operation; however, thesignal control chip may be configured by a plurality of chips which areinterconnected by external wirings. In the present embodiment, thejudgment whether or not the picture is a moving picture is performedimmediately after the signal processing unit 110 receives the imagesignal from the image source unit 100; however the judgment is notlimited at this timing. More specifically, the judgment may be performedat any signal path because the judgment is performed for the image databetween the frames based on the ratio of the pixels having a changebetween the frames. In the above embodiment, the second calculationsection 504 acts as a moving-picture judgment section 114; however, thefirst calculation section 502 for processing the monochromatic imagedata may act as the moving-picture judgment section 114. In analternative, the moving-picture judgment may be performed using both thefirst calculation section 502 and second calculation section 504. In afurther alternative, timing controllers 505 and 506 may perform themoving-picture judgment.

In the present embodiment, the moving-picture judgment is performedbased on the signal change between the frames. However, if the picturehas a movement in the pixels, a specific color signal or brightnessthereof may be used for the moving-picture judgment because the movementin the pixel is accompanied by a brightness change at any time. In thecase wherein the moving-picture judgment is performed based on thespecific color signal or brightness thereof the memory capacity neededfor the judgment can be reduced compared to the case wherein the entirepicture is compared between the frames, thereby reducing the cost neededfor the judgment. The moving-picture judgment section 114 need not beinstalled within the signal control chip 112, and may be separatelyprovided outside the signal control chip 112. In this case, it issufficient that only the control signal is output in synchrony with thesignal from the control-signal generating section 507.

In the present embodiment, when the scanning direction is oppositebetween the first LC panel 133 and the second LC panel 136, the scanningis started at the bottom row in the first LC panel 133 and at the toprow in the second LC panel 136. However, the starting position for thescanning may be selected as desired, and thus the starting position isnot limited to the top row or bottom row. The starting positions are notlimited to the top row and bottom row in both the LC panels that aresymmetric to each other with respect to a central row. For example, ascanning line that is five line above the bottommost scanning line maybe used as the starting row in the first LC panel 133, whereas ascanning line that is three lines below the topmost scanning line may beused as the starting row in the second LC panel 136. In a furtheralternative, the scanning may be started at the same scanning line inboth the first ad second LC panels, and the direction of the scanning isopposite in both the first and second LC panels.

In the present embodiment, the scanning direction is opposite betweenthe first LC panel 133 and the second LC panel 136. This configurationreduces the flicker in the stacked LCD unit including a plurality of LCpanels stacked one on another. In addition, the picture to be displayedon the LCD unit is subjected to moving-picture judgment as to whetherthe picture is a moving picture or a still picture. If the picture is astill picture, opposite scanning directions are employed in the firstand second LC panel 133, 136, whereas if the picture is a movingpicture, the same scanning direction is employed in the first and secondLC panels 133, 136. This reduces the sense of discomfort in the observerduring display of a moving picture, while reducing the influence by theflicker during display of a still picture.

A LCD unit according to a second embodiment of the present inventionwill be described hereinafter again with reference to FIG. 1. In thefirst embodiment, the moving-picture judgment section 114 judges whetherthe picture is a moving picture or a still picture based on the changeof image data occurring between frames, to switch the operation mode ofthe LCD unit 130. In the present embodiment, the image source unit 100supplies information specifying whether the picture delivered from theimage source unit 100 is a moving picture or still picture, whereby theLCD unit 130 switches the operation mode based on the deliveredinformation. The other configurations are similar to those in the firstembodiment.

In the second embodiment, it is assumed here that the image source unit100 is a personal computer, on which an application such as action gameprogram or video player program is started to run. In this case, most ofthe data delivered from the image source unit 100 is a moving picture.The image source unit 100 detects that a specific application programproviding a moving picture is started to run on the computer, to delivera moving-picture flag to the signal processing section 110. Themoving-picture flag may use a signal line such as USB (universal serialbus) or RS232C provided separately from the other signal lines, or mayuse the signal cable 103 for transmission together with the image data.

The signal control chip 112, upon absence of the moving-picture flag,determines the opposite scanning directions for the first LC panel 133and the second LC panel 136. The signal control chip 112, uponoccurrence of the moving-picture flag, judges that reproduction of amoving picture is started and determines the same scanning direction forthe first LC panel 133 and the second LC panel 136. This configurationallows the LCD unit 130 to use the same scanning direction for the firstLC panel 133 and second LC panel 136, if reproduction of a movingpicture program is started to decode the moving picture of compresseddata such as MPEG data.

In the present embodiment, the signal processing unit 110 receives, fromthe image source unit 100, information (flag) that specifies whether ornot a specific application program that mostly delivers a moving pictureis started, and changes the scanning direction of the first LC panel 133based on the received flag. This configuration allows the first LC panel133 and second LC panel 136 to employ opposite scanning directions andthus reduce the flicker, during reproduction of a still picture. Theconfiguration also allows the first LC panel 133 and second LC panel 136to employ the same scanning direction during reproduction of the movingpicture, ad thus reduces the sense of discomfort perceived by theobserver.

A LCD system according to a third embodiment of the present inventionwill be described hereinafter. The above embodiments premise that thelight source 137 (FIG. 1) includes a cold cathode fluorescent light(CCFL) or light emitting diode (LED) that emits a white and uniformlight. In the present embodiment, the light source emits RGB lights byusing a time-sharing scheme. The stacked LC panels of the LCD unit 130each display a picture in a field sequential mode by using atime-sharing scheme. The processing in the first LC panel 133 and secondLC panel 136 for generating image data is similar to that in the firstexemplary embodiment. Moreover, the opposite scanning directions areused in the first and second LC panels during display of a still picturefor reducing the flicker in the present embodiment as well. Upon displayof a moving picture, the same scanning direction is used also in thefirst and second LC panels for reducing the sense of discomfort in theobserver. Thus, the advantages of the present embodiment are similar tothose in the first ad second embodiments. A color picture is preferablydisplayed on the LCD unit of the present embodiment.

A LCD system according to a fourth embodiment of the present inventionwill be described hereinafter. In the present embodiment, the LCD unitis driven in a drive mode such as a TN mode wherein the voltage appliedto the LC layer changes the angle of LC molecules with respect to thesubstrate surface. Such a drive mode generally suffers from the problemof viewing angle dependency wherein the viewing angle range allowing thepicture to be observed depends on the viewing direction of the observerwith respect to the LCD unit. This problem results from the fact thatchange of the angle of LC molecules with respect to the substratesurface varies birefringence characteristic of the LC moleculesdepending on the viewing direction of the observer. When a plurality ofsuch LC panels are stacked one on another, it is considered thatdegradation of the image quality caused by the viewing angle dependencyincreases depending on the number of the stacked LC panels providing asynergetic effect. For avoiding the synergetic effect in the presentembodiment, adjacent two, for example, of the stacked LC panels areallowed to have opposite viewing angle dependencies. The oppositeviewing angle dependencies cancel each other in the adjacent LC panels,whereby the viewing angle range is averaged for the viewing directions.

The above embodiments use TFTs as three-terminal non-liner devices fordriving the pixel electrodes in each LC panel; however the non-lineardevices are not restricted to the TFTs. For example, the non-lineardevices may be two-terminal devices such as thin-film diodes (TFDs). Inan alternative, the LC panels may be driven in a simple-matrix drivescheme without using the non-linear devices so long as the LCD unit hasa relatively lower resolution. The LCD units according to the aboveexemplary embodiments achieve the advantage of higher contrast ratio,whereby the LCD units can be preferably used as a image display unit ofa diagnostic imaging device for which a higher contrast ratio isdesired, a monitor used in a broadcasting station, a LCD unit of anelectronic equipment used in a dark room, such as a movie theater.

A LCD unit according to a fifth embodiment of the present invention,will be described hereinafter. In the first embodiment, the same canningdirection used for display of a moving picture in the first LC panel 133and second LC panel 136 cannot achieve the advantage of reduction in theflicker. In the present embodiment, the same scanning direction is usedfor reducing the flicker by employing a configuration wherein thestarting row for staring the scanning in the first LC panel 133 isadjusted with respect to the starting row in the second LC panel 136.More specifically, a topmost row (scanning line) is selected as thestarting row of the first LC panel 133, whereas a central row isselected as the starting row in the second LC panel 136. In thisconfiguration, each of the first and second LC panels 133, 136 selectsconsecutively the own scanning lines with a specific space beingmaintained from the scanning line of the other of the first and secondLC panels 133, 136.

As described in connection with the first embodiment, the initialvoltage Vs of each pixel of the first LC panel 133 and second LC panel136 immediately after writing data into the each pixel is expressed by:Vs=Qs/C1,whereas the final voltage Ve of the each pixel immediately beforewriting data in the next frame is expressed by:Ve=(Qs−Qr)/C1.Assuming that the voltage drop from Vs to Ve as described above islinear with respect to time, when a pixel of the first LC panel 133 hasa midpoint potential between Vs and Ve, i.e., (Vs+Ve)/2, thecorresponding pixel of the second LC panel 136 has a potential of Vsbecause of the difference in the starting row of the scanning.Similarly, when a pixel of the second LC panel 136 has a midpointpotential between Vs and Ve, the corresponding pixel of the first LCpanel 133 has a potential of Vs. The potential difference between thepixel of the first LC panel and the corresponding pixel of the second LCpanel is suppressed to half the potential difference Vs−Ve, which thecase of scanning at the same starting row provides.

The scanning scheme of the present embodiment reduces the potentialdifference between the corresponding pixels of the first and second LCpanels down to half the potential difference that the same starting rowprovides between the corresponding pixels of the first and second LCpanels. This allows each pixel of the LCD unit to have a range ofvariation in the light intensity which is half the range of variation inthe light intensity that the same starting row provides. Moreover, thefrequency of the flicker is doubled up to 120 Hz, if the drive frequencyis 60 Hz, for example. A higher flicker frequency reduces the sense ofdiscomfort caused by the flicker in the observer.

In the present embodiment, as described above, the starting row forscanning in each frame is differentiated between the first LC panel 133and the second LC panel 136, thereby reducing the range of variation inthe light intensity between corresponding pixels and raising the flickerfrequency to reduce the sense of discomfort caused by the flicker. Inthe present embodiment, the same scanning direction is employed for thefirst LC panel 133 and second LC panel 136. However, the oppositescanning directions may be employed in association with the differentstaring rows. This reduces the flicker frequency to suppress the senseof discomfort caused by the flicker.

In the above embodiments, the second calculation section 504 created24-bit color image data from the 8-bit image data for each RGBsub-pixels. The number of bits of the input data and output data is notlimited to this example. For example, if it is assumed that the numberof gray-scale levels of each LC panel in a plurality (n) of stacked LCpanels is m, then the maximum number of gray-scale levels represented bythe LCD unit is m×n. Thus, the present invention may employ aconfiguration wherein the number of gray-scale levels in the input imagedata is between “m” and “m²” inclusive of both, and the secondcalculation section 504 creates color image data having m gray-scalelevels from the input image data.

In the above embodiments, the LCD unit 130 includes two LC panels, forthe sake of simplification for the description. However, the LCD unitmay include three or more LC panels. If a plurality (n, which is equalto or larger than two) of LC panels is used, a single color LC panel ispreferably provided therein. The single color LC panel reduces the rangeof variation in the brightness of transmitted light even if the viewingdirection is changed, compared to the case where a plurality of color LCpanels are provided.

If the LCD unit 130 includes a plurality (n, which is equal to or largerthan two) of LC panels, at least one LC panel among them preferably hasa scanning direction opposite to the scanning direction of the remainingLC panels. If the LCD unit includes three LC panels, for example, aconfiguration may be employed wherein a front LC panel and a rear LCpanel are scanned upward for example, and the central LC panel isscanned downward to reduce the flicker.

While the invention has been particularly shown and described withreference to exemplary embodiment and modifications thereof, theinvention is not limited to these embodiment and modifications. It willbe understood by those of ordinary skill in the art that various changesin form and details may be made therein without departing from thespirit and scope of the present invention as defined in the claims.

1. A liquid crystal display (LCD) unit comprising: a plurality of liquidcrystal (LC) panels each including an array of pixels, said LC panelsbeing stacked one on another so that corresponding pixels of said LCpanels are aligned with one another for display of a picture; and abacklight source emitting light onto a rear side of said stacked LCpanels, wherein a plurality of rows of said pixels of at least one ofsaid LC panels are scanned consecutively in a first scanning directionfrom a first row of said pixels, and a plurality of said pixels of restof said LC panels are consecutively scanned in a second scanningdirection from a second row of said pixels, and wherein i) said firstscanning direction is the same as said second scanning direction duringdisplay of a moving picture, and ii) said first scanning direction isopposite to said second scanning direction during display of a stillpicture.
 2. A liquid crystal display (LCD) unit comprising: a pluralityof liquid crystal (LC) panels each including an array of pixels, said LCpanels being stacked one on another so that corresponding pixels of saidLC panels are aligned with one another for display of a picture; abacklight source emitting light onto a rear side of said stacked LCpanels, wherein, a plurality of rows of said pixels of at least one ofsaid LC panels are scanned consecutively in a first scanning directionfrom a first row of said pixels, and a plurality of said pixels of restof said LC panels are consecutively scanned in a second scanningdirection from a second row of said pixels, and said first scanningdirection is opposite to said second scanning direction, and/or saidfirst row is different from said second row; and a mode switchingsection for selecting one of a first mode wherein a plurality of rows ofsaid pixels of said LC panels are scanned consecutively in the samedirection from the same row, and a second mode wherein a plurality ofrows of said pixels of at least one of said LC panels are scannedconsecutively in a first scanning direction from a first row of saidpixels, a plurality of said pixels of rest of said LC panels areconsecutively scanned in a second scanning direction from a second rowof said pixels, and said first scanning direction is opposite to saidsecond scanning direction and/or said first row is different from saidsecond row.
 3. The LCD unit according to claim 2, wherein said modeswitching section switches between said first mode and said second modeat an interval between two of writing periods for said pixels.
 4. TheLCD unit according to claim 2, further comprising a moving-picturejudgment section for judging whether the picture is a still picture or amoving picture, wherein said mode switching section selects said firstmode if said moving-picture judgment section judges that the picture isthe moving picture, and selects said second mode if said moving-picturejudgment section judges that the picture is the still picture.
 5. TheLCD unit according to claim 4, wherein said moving-picture judge sectionperforms said judgment based on a ratio of number of pixels for whichimage data is changed to a total number of said pixels.
 6. The LCD unitaccording to claim 5, wherein said moving-picture judgment sectionjudges that the picture is the moving picture if said ratio exceeds afirst threshold, and judges that the picture is the still picture ifsaid ratio is below a second threshold which is lower than said firstthreshold.
 7. The LCD unit according to claim 6, wherein saidmoving-picture judgment section judges that the picture is the movingpicture if a state where said ratio exceeds said first thresholdcontinues a first time period, and judges that the picture is the stillpicture if a state where said ratio is below said second thresholdcontinues a second time period.